media: i2c: max9286: fix access to unallocated memory

The asd allocated with v4l2_async_notifier_add_fwnode_subdev() must be
of size max9286_asd, otherwise access to max9286_asd->source will go to
unallocated memory.

Fixes: 86d37bf3

 ("media: i2c: max9286: Allocate v4l2_async_subdev dynamically")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Cc: stable@vger.kernel.org # v5.10+
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>